1. Field of the Invention
The present invention relates to a method for performing a burn-in test which is an effective conductivity test for screening elements. In particular, the present invention relates to a method for performing a burn-in test for a light source unit that is provided with a light source for thermally-assisted magnetic recording.
2. Description of the Related Art
In the field of magnetic recording using a head and a medium, further improvements have been demanded in performance of thin film magnetic heads and magnetic recording media in view of an increase in recording density of magnetic disk devices. For the thin film magnetic heads, composite type thin film magnetic heads configured from lamination of a reading magnetoresistive (MR) element and a writing electromagnetic conversion element are being widely used.
The magnetic recording medium is a non-continuous medium, in which magnetic particles are aggregated. Each magnetic particle has a single magnetic domain. In this magnetic recording medium, a single recording bit is configured by a plurality of magnetic particles. Therefore, to increase magnetic density, the size of the magnetic particles must be reduced, and asperity at a border of adjacent recording bits needs to be minimized. However, if the size of the magnetic particles is reduced, there is a problem that thermal stability for magnetization of the magnetic particles is lowered as the volume of the magnetic particles is reduced.
To address this problem, increasing magnetic anisotropic energy Ku of magnetic particles may be considered. However, this increase in Ku causes an increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. On the other hand, the upper limit of the writing magnetic field intensity for the thin film magnetic head is determined substantially by saturation magnetic flux density of a soft magnetic material forming a magnetic core in the head. As a result, when the anisotropic magnetic field of the magnetic recording medium exceeds an acceptable value determined from the upper value of the writing magnetic field intensity, writing becomes impossible. Currently, as a method to solve such a problem of thermal stability, a so-called thermally assisted magnetic recording method has been proposed, which, using a magnetic recording medium formed by a magnetic material with large Ku, performs the writing by heating the magnetic recording medium immediately before applying the writing magnetic field to reduce the anisotropic magnetic field.
For this thermally assisted magnetic recording method, a method that uses a near-field light probe, a so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmon excited by emission of laser light, is known.
With this type of thermally-assisted magnetic recording, a major issue is where and how a high output light source is installed in a magnetic head in order to provide sufficiently high intensity light stably at a desired position.
Concerning placement of the light source, U.S. Pat. No. 7,538,978 B2 discloses a structure that contains a laser unit including a laser diode on a rear side of a slider. US Patent Publication No. 2008/0056073 A1 discloses a structure in which a structural body is mounted on an edge surface (or rear side) of a slider, the structural being that a reflective mirror is monolithically integrated on a laser diode element.
Furthermore, the inventors of the present application propose a thermally-assisted magnetic recording head for a so-called “composite slider structure” that is configured to connect a light source unit provided with a light source to an end surface (back surface) on a side opposite to an air bearing surface (ABS) of a slider that provides a magnetic head element.
This type of “composite slider structure” is disclosed, for example, in US Patent Publication No. 2008/0043360 A1 and US Patent Publication No. 2009/0052078 A1.
Furthermore, a thermally-assisted magnetic recording head of a “composite slider structure” has the following advantages (1)-(4):
(1) The air bearing surface and the integrated surface in the slider are perpendicular, and therefore have good compatibility to a conventional thin-film magnetic head manufacturing process;
(2) The light source can be far from the air bearing surface, and conditions that cause direct mechanical impact to the light source during the operation can be avoided;
(3) Because optical components that require extremely high precision, such as an optical pickup lens, and optical components that require a special structure for connecting, such as optic fiber or the like, are not required within the head, the number of manufacturing steps can be reduced, and cost will be lower; and(4) With regards to a property evaluation and a reliability evaluation during the manufacturing process, the laser diode, which is the light source, and the magnetic head element can be independently evaluated. As a result, a situation can be avoided where the yield for the light source and the yield for the slider have an additive effect, such as when the light source and the magnetic head element are all provided within the slider, and cause the yield of the entire head to dramatically decrease.
Herein, a reliability evaluation for a light source unit provided with a laser diode is particularly considered. Performing a burn-in test is effective for reliability evaluation of this type of light source. Herein, a burn-in test is a test for screening test subjects by electrifying a test subject (herein a laser diode provided on a light source unit), measuring the over time change of one property of the test subject at high temperature (for example, under heating conditions at 80° C.) while maintaining the electrification, and evaluating the over time change of the property.
However, this type of burn-in test requires an extremely long time, such as from several hours to several tens of hours, in order to evaluate a single laser diode.
Therefore, as a countermeasure, evaluating a plurality of laser diodes in parallel in a bar condition prior to cutting and separating into individual light source chips in the light source manufacturing process is very effective. By performing this type of parallel processing, a burn-in test for a large number of laser diodes can be performed at one time, and the number of evaluation processes and the time can be greatly reduced.
However, simultaneously contacting a power supply probe to a large quantity of electrodes for a plurality of laser diodes placed in a bar is extremely difficult. Furthermore, even if a power supply probe can simultaneously contact with a large quantity of electrodes for a plurality of laser diodes as a result of an innovation in the configuration of the bar or the like for example, the following problems occur.
Namely, if the bar length is 80 mm, for example, it is thought that 100 to 200 laser diodes (LD chips) can be present in a single bar.
A laser diode having a light output of approximately 100 mW is typically used. Herein, the input power is generally approximately 3 times the light output, and in this case, 200 mW is converted to Joule heat. Assuming that 100 elements are simultaneously made to emit, an amount of heat corresponding to 20 W is concentrated in a bar with a small volume that contains the LD chips.
If such heat is not effectively dissipated to a fixture that contacts with the bar, the deviate of the LD chip temperature with regards to the test environment temperature is severe, and problems occurs in that an accurate evaluation is not possible. There is concern that, in the worst case, large thermal stress is applied to the LD chip, and the chip itself is destroyed.
Therefore, it is conceivable to make structural improvements in order to dissipate the heat to the fixture that contacts the bar. However, the structure of the fixture that incorporates the bar or the like becomes complex.
The present invention is conceived based on this situation, and an object thereof is to propose a method for performing a burn-in test and a test device that maintains a simple device structure while holding a stable temperature in a short period of time and maintaining a temperature that does not deviate from normal load conditions, and that performs a sorting test between defect parts and good parts for light source unit chips without causing damage to the element.